Update background color of selected Tree View item to meet contrast ratio requirement

Describe the bug

Selected TreeViewItems do not currently meet the 3:1 contrast ratio requirement relative to unselected items, as specified in MAS 1.4.11 - Non-text Contrast.

Light theme ratio is 1.5:1

Dark theme ratio is 1.3:1

Additional context

This issue is closely related to #2908 but requires different theme resources to be updated.

Proposed fix Create new resources

<RevealBackgroundBrush x:Key="SystemControlHighlightAccent1RevealBackgroundBrush" TargetTheme="Dark" Color="{ThemeResource SystemAccentColorDark1}" FallbackColor="{ThemeResource SystemAccentColorDark1}" />
<RevealBackgroundBrush x:Key="SystemControlHighlightAccent1RevealBackgroundBrush" TargetTheme="Light" Color="{ThemeResource SystemAccentColorLight1}" FallbackColor="{ThemeResource SystemAccentColorLight1}" />

Update TreeViewItemBackgroundSelected to use the new SystemControlHighlightAccent1RevealBackgroundBrush.

[Felix-Dev]

Yep, I am asking @YuliKl for this reason (and consistency reasons) to consider switching the currently used TreeViewItem selected brushes to the ones used by the ListViewItem. If we won’t do that, @dpaulino, you can always override those brushes yourself as shown in that issue (while waiting for a fix which would presumably have to wait until WinUI 3).

[YuliKl]

Closing this issue. TreeView styles and colors are being updated in the near future.